Set post-thumbnail (with php)?

Question

The issue is going to be the difference in the data required for setting the post thumbnail (an attachment ID) and what’s probably in the custom field: a URL.

The way I’m going to suggest is very sloppy: adds more attachments to WordPress. A better way might be to write a custom SQL query where you grab the attachment ID from the custom field URL. This works, it just might not be the best way.

First, get all the posts:

// get an array of all the posts
    $posts = get_posts(
        array( 
            'numberposts'   => -1,
            'post_type'     => 'post'
        )
    );

    // bail if our get_posts() call failed
    if( empty( $posts ) ) return;

Then we’ll loop through them, grabbing the original Image URL, then inserting it as a new attachment. Finally, we’ll update the postmeta table to reflect our new thumbnail ID and get rid of the old post thumbnail custom field.

foreach( $posts as $p )
    {
        // change your old thumbnail key!
        $image_url = get_post_meta( $p->ID, 'your_old_thumbnail_key', true );

        // no thumbnail? on to the next
        if( empty( $image_url ) ) continue;

        // find our mime type for later
        $filetype = wp_check_filetype( $image_url );

        // Set up an array of args for our new attachment
        $args = array(
            'post_mime_type' => $filetype['type'],
            'post_title'     => esc_attr( $p->post_title ), // you may want something different here
            'post_content'   => '',
            'post_status'    => 'inherit'
        );

        // Insert the attachment!
        $thumb_id = wp_insert_attachment( $args, $image_url,  $p->ID );

        // gotta set up some meta data (height, width, etc)
        // our functions are int his file, so we have to include it
        require_once(ABSPATH . 'wp-admin/includes/image.php');
        $metadata = wp_generate_attachment_metadata( $thumb_id, $image_url );
        wp_update_attachment_metadata( $thumb_id, $metadata );

        // Finally! set our post thumbnail
        update_post_meta( $p->ID, '_thumbnail_id', $thumb_id );

        // Uncomment the next line to delete the old custom field
        //delete_post_meta( $p->ID, 'your_old_thumbnail_key', $image_url );
    }

That’s it!

Here it is as a plugin: http://pastie.org/2386814

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